The selective oligomerization of isobutene is an important chemical reaction, particularly where the isobutene is contained in a refinery C4 hydrocarbon stream, such as Raffinate-1 and Raffinate-2.
For example, C4 linear olefins are an attractive feedstock for producing octenes with zeolite catalysts because, among other reasons, the resulting octenes have triple branching of less than about 5 wt %. However, when isobutene is present in significant quantities (>10 wt %) in the oligomerization feedstock, such as with Raffinate-1, the amount of triple-branched octenes increases to a level which is unacceptable for some end uses such as certain plasticizers. In the past, this problem was generally addressed by selectively reacting the isobutene with methanol to produce methyl t-butyl ether (MTBE). However, with the phase-out of MTBE because of environmental concerns, this reaction is no longer an attractive method of removing isobutene. As a result, interest has focused on selective dimerization of the isobutene to produce octenes useful as, for example, gasoline octane enhancers and as feedstocks for producing C9 aldehydes and/or alcohols.
Another use for the selective oligomerization of isobutene is in the purification of C4 olefin streams used in the alkylation of benzene to produce sec-butylbenzene, an important precursor in the production of phenol. Thus, even when present at only low levels (<5 wt %) in a C4 olefin stream, such as Raffinate-2, isobutene reacts with benzene to produce tert-butylbenzene. However, tert-butylbenzene is difficult to separate from sec-butylbenzene by distillation since the boiling points of the two butylbenzene isomers are very similar, 169° C. for tert-butylbenzene as compared with 173° C. for sec-butylbenzene. Moreover, tert-butylbenzene is known to be an inhibitor to the oxidation of sec-butylbenzene to the corresponding hydroperoxide, which is the first step in the conversion of sec-butylbenzene to phenol and methyl ethyl ketone.
One example of a process for the dimerization of isobutene is disclosed in U.S. Pat. No. 6,914,166, in which a C4 olefinic feedstock containing isobutene and n-butene(s) is contacted with dealuminated zeolite beta under conditions including a temperature below 50° C. effective to allow selective dimerization of isobutene to trimethylpentene(s). However, although this process appears to be highly selective for isobutene conversion, the catalyst is found to age rapidly at the conditions employed.
U.S. Pat. No. 6,274,783 discloses a process for the concurrent dimerization and hydrogenation of isobutene in a single distillation column reactor containing both a dimerization catalyst, such as a zeolite or an acidic cation exchange resin, and a hydrogenation catalyst, such as a Group VIII metal deposited on a carrier or support. There is no indication that the process would selectively dimerize isobutene in a mixed C4 olefin feed.
U.S. Pat. No. 6,500,999 discloses a process for the production of hydrocarbons with a high octane number starting from hydrocarbon cuts containing isobutene by means of selective dimerization with acid catalysts, wherein the dimerization reaction is carried out in a tubular reactor using a feed containing isobutene in quantities of less than 20% by weight and with a molar ratio of linear olefins/isobutene greater than 3, preferably operating at a reaction temperature ranging from 30 to 120° C., at a pressure of less than 5 MPa and at feed space velocities of less than 60 hr−1. Suitable acid catalysts are said to include phosphoric acid supported on a solid carrier, a cationic acid exchange resin, a liquid acid, a sulfonic acid derivative, a silico-alumina, a mixed oxide, a zeolite or a fluorinated or chlorinated alumina.
U.S. Pat. No. 7,112,711 discloses a process for oligomerizing alkenes having from 3 to 6 carbon atoms, including isobutene, in the presence of a catalyst containing a zeolite of the MFS structure type. The process is carried out at a temperature comprised between 125 and 175° C. when the feedstock contains only alkenes with 3 carbon atoms and between 140 and 240° C. when the feedstock contains at least one alkene with 4 or more carbon atoms
U.S. Patent Application Publication No. 2007/0213576, published Sep. 13, 2007, discloses a process for the dimerization of isobutene at a temperature in excess of 240° C. in the presence of a multi-dimensional molecular sieve catalyst containing at least one 10 or 12 ring channel, such as ZSM-57, ZSM-5, FAU, Beta, ZSM-12, mordenite, MCM-22 family zeolites, and mixtures thereof to produce a product low in triple-branched octenes.
K. Hauge et al in Catalysis Today, Vol. 100 (2005) pp. 463-466 reported on the oligomerization of pure isobutene over solid acid catalysts at 40° C. and 10 bar pressure and 60 WHSV. The zeolite solid acid catalysts (ZSM-5, beta, and Y) had high initial activity, but the activity dropped drastically over the course of 2 hours. This was attributed to “the production of high molecular weight oligomers inside the zeolite pores.” The macroreticular acidic resin Amberlyst 15, in contrast, showed activity that leveled out after about 2 hours.
According to the present invention, it has now been found that, when operated at low temperature, molecular sieves of the MCM-22 family are selective and stable catalysts for the oligomerization of isobutene in the presence of other C4 alkenes. Although the reason for this result is not understood, it is believed that the presence of the active acid sites in pockets on the external surface of the zeolite crystal in MCM-22 molecular sieves allows the reaction products to quickly desorb, thereby avoiding the production of high molecular weight oligomers inside the zeolite pores as reported by K. Hauge et al in Catalysis Today, Vol. 100.